This invention relates to heat-curing reactive systems comprising mixtures of polyethers and polyesters that are not homogeneously miscible with one another, optional OH-and/or NH.sub.2 -terminated low molecular weight chain extenders, optional auxiliaries, and a solid polyisocyanate having a melting point above 80.degree. C., in which the polyester component is present in admixture as a solid (such as powder or granules).
Polyurethanes based on polyoxyalkylene ethers are known to have poorer mechanical properties than polyester-based polyurethanes. Additional intermolecular secondary valence bonding forces occur in polyester urethanes by virtue of the polar groups present. A consequence of this is the crystallinity exhibited by many polyesters, which ultimately determines the quality of the end product. Thus, polyesters --or rather polyester urethanes --often behave differently from polyether systems, even at high or low temperatures. Polyester polyurethanes are used as high quality products, particularly in those industrial fields where stringent mechanical requirements must be satisfied.
Nevertheless, the majority of polyurethanes are based on polyethers. Polyethers can be "refined" by modification in numerous ways, so that the resulting end products also satisfy more stringent practical requirements. Another important factor is that polyethers are generally liquid at room temperature and, hence, are much easier to process than, for example, solid polyesters. Other advantages and disadvantages of both classes of these high molecular weight compounds are well known to one skilled in the art. There has been no shortage of attempts to eliminate or at least reduce the disadvantages specific to polyethers or polyesters by addition of polyesters or polyethers. Unfortunately, many polyethers and polyesters are incompatible with one another because of differences in their structural units. Emulsions or suspensions which separate very quickly into two phases are obtained after mixing of the two components, particularly when using polyesters based, for example, on adipic acid and ethylene glycol or 1,4-butanediol.
When used in combination with chain-extending agents and polyisocyanates, commercially available, inexpensive polyesters give particularly high-quality elastomers. A two-phase system is particularly difficult to process on an industrial scale. Despite appropriate countermeasures (for example, use of special stirrers), dosing errors can occur. In addition, phase separation occurs during solidification, even after addition of the polyisocyanate. These more or less distinct phases react with the isocyanate after a relatively long reaction time, but a homogeneous, complete reaction often does not occur. Because polyesters are generally much more reactive than polyethers, preliminary reactions occur. The more sluggishly reacting polyether can often appear as a "greasy layer" on the surface of the moldings. The poly-addition reaction can be accelerated by the use of suitable catalysts. Because of a rapid increase in viscosity, however, the "pot life" of the reaction mixtures is affected, so that processing by casting is not possible. It has now surprisingly been found that the disadvantages mentioned above do not arise when a powder-form polyester that is solid at room temperature is suspended as the polyester component in the polyether.
The resultant suspensions are pourable, spreadable, or paste-like, depending on the quantity of solids added, and o solidify upon heating. It has surprisingly been found in this regard that the ordinarily incompatible polyethers and polyesters do not separate. Instead, homogeneously cured moldings having a "dry surface" are obtained after rapid hardening.
Microscopic examination of thin films has shown that the polyurethane matrix consists of a polyether urethane or a polyester urethane, depending on the polyether or polyester component. When a matrix of polyether urethane is present, the s polyester urethane is uniformly distributed throughout the polyether urethane matrix as small beads (about 1-5 .mu.m). In When elongated, these beads are longitudinally deformed in the direction of the force applied. After relaxation, the starting condition is re-established.